Trailer Lights Troubleshooting

Downtime is money, and when your trailer lights are giving you troubles, your trailer is not road legal. When it is sitting in the garage, you are losing time and money. Use the following general guidelines on trailer lights troubleshooting that will help you find and solve the issue and get your trailer back on the road with its lights performing exactly as expected.

Trailer light problems can be traced to any point in either the tow vehicle’s electrical system or trailer wiring, so visual inspection and testing are the first things to do. Because the most common reason for trailer lights not working properly is a bad or missing ground, check the trailer’s and truck’s ground connection first. A poor ground often becomes the culprit when only some lighting functions are working and when lights go out when you push the brake pedal. This means that using several functions at the same time creates the load that a weak ground can’t handle. Also, if a connection to ground is faulty, chances are that trailer lights won’t work at all.

Check ground for the following issues:

Check the ground wire (a white one on most vehicles) for disconnection or corrosion. Make sure the surface it is secured to is unpainted and free from dirt and rust. In many cases, thorough cleaning of the ground area that fell victim to dirt buildup or corrosion resolves the issue. Check the area the ground is secured to – it should be the bare metal frame. A ground connection can also loosen over time.

Trailer tail lights not working? Make sure that each of tail lights is properly grounded. Ground is achieved in one of two ways. In the first case, separate wires come out of each tail light assembly and are attached to a metal frame. A more common case scenario presupposes ground to be provided through the bolts of the tail light assembly. Tail light housings should be secured to a metal surface (trailer frame), not wood or plastic.

Use a circuit tester to check whether current is flowing through.

The cause for only some of lighting functions working may also lie in “hot” wire issues. Use the circuit tester to see if current is going to the light fixture itself. Begin with checking the harness that goes into the fixture in question and move all the way back to the tow vehicle to find the point where the circuit breaks. You may need to clean the brass terminals using steel wool or wire brush to restore a good connection.

To find the reason for trailer lights problem, you may need to check a fuse box on a tow vehicle for blown fuses. If you have a power converter or a T-connector, disconnect the trailer and use a circuit tester to check whether a signal is going into and back from the converter or Modulite box on the appropriate wires. If there is no signal output going into the box, the problem is on the tow vehicle’s side (again, it can be caused by a blown fuse or wrong connecting points). If the output is going into the box without coming out at all or goes on the wrong wires, check the grounding location. If the output is going in and back on the right wires, the reason may be wrong trailer wiring, including improper ground wire connection, dirt or rust buildup, pinched or cut harnesses.

Troubleshooting trailer lights can appear to be intricate, but a good visual inspection can take a great deal of problems off. You have to thoroughly examine all connections, light fixtures, and bulbs to find the reason for trailer lights not working. To avoid false ground from the tow vehicle to the trailer, make sure that the trailer hitch and the tow vehicle ball, as well as safety chains or cables are disconnected. Jack and plug should be the only connection between a truck or SUV and a trailer.

About Andrew Demaree

Andrew Demaree is Bully’s resident technical writer and automotive expert. Being SAE certified mechanic, Andrew knows how your vehicle works and how to make it run even smoother. He is also passionate about truck customization and off-roading, so when it comes to adding some style and functionality to your truck, you can rely on Andrew’s knowledge.